Alkylation encountering acidity run-aways

ABSTRACT

Run-away sulfuric acid of less than catalytic strength (88 percent), or the entire reaction mixture containing such acid, is charged to a normally operating alkylation reaction, and the acid is thus restored to alkylation strength. The alkyl sulfate which would contaminate the alkylate product of a run-away acid is thus alkylated resulting in a sulfur-free alkylate.

United States Patent [151 3,683,041 Goldsby [451 Aug. 8, 1972 [54] ALKYLATION ENCOUNTERING 3,055,958 9/1962 Webb, Jr. ..260/683.62 ACIDITY RUN-AWAYS 3,534,] 18 10/1970 Massa ..260/683.6l [72] Inventor: Arthur R. Goldsby, Chappaqua, 3,038,948 6/1962 Trow ..260/683.62

FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Texaco Development Corporation, 677,128 12/1963 Canada ..260/683.59

New York, N.Y.

Primary Examiner-Delbert E. Gantz [22] Flled' 1969 Assistant Examiner--G. J. Crasanakis [21] Appl. N0.: 889,476 Attorney-Thomas H. Whaley and Carl G. Ries [52] U.S. Cl ..260/683.59, 260/683.62 [57] ABSTRACT [51] Int. Cl ..C07c 3/54 y sulfuric acid of less than catalytic Strength 581 Field of Search ..260/683.59, 683.61, 683.62, Percent), of the entire reaction mixture Contain- 260/683 46 ing such acid, is charged to a normally operating alkylation reaction, and the acid is thus restored to al- [56] Reerences Cited kylation strength. The alkyl sulfate which would contaminate the alkylate product of a run-away acid is UNITED STATES PATENTS thus alkylated resulting in a sulfur-free alkylate. 3,007,983 11/1961 Clauson ..260/683.61 2 Claims, 1 Drawing Figure Aim-away fear/12'? MHZ/'6 1 ALKYLATION ENCOUNTERING ACIDITY RUN- AWAYS BACKGROUND OF THE INVENTION 1. Field of the Invention When an olefin hydrocarbon is being alkylated with an isoparaffin using a sulfuric acid catalyst the acid sometimes and without warning starts dropping in acidity very rapidly. This is spoken of as a run-away, and the resulting acid as run-away acid. For a given set of alkylation conditions there is a minimum alkylation acidity. If the acidity of the system acid drops below this figure, the alkylation reaction ceases and the acidity of the acid drops rapidly. If a run-away is not detected almost immediately, the acidity will drop so fast, and so far, that it becomes necessary to remove the acid from the system. At the same time the product alkylate usually becomes contaminated with sulfur compounds in the form of alkyl sulfates so that it has a reduced or negative lead susceptibility. Thus, when. such a condition occurs, acid and alkylate must be discarded and therefore are lost, or they must be further processed to make them suitable for use.

2. Description of the Prior Art It is advantageous to run a sulfuric acid alkylation plant with the system acid as near to the minimum acidity as possible since this type of operation results in the lowest acid consumption. However, some alkylation process operators feel that running in such a manner tends to promote corrosion, and also tends to result in more run-aways of the acid. As a result, most operators try to keep modestly above the minimum acidity for the system acid, for example, acidities of at 90-92 percent rather than at 88-90 percent. Unfortunately, the minimum acidity varies with other reaction conditions and all of the factors affecting the minimum acidity are not at all well understood. Many run-aways have not had an adequate explanation. It is generally thought they can be and are caused by a large variation of operating conditions, such as a decrease in mixing efficiency, increase in sulfur content of feed, increase in olefin feed rate, increase in water content, decrease in isobutane concentration, and an increase in ethylene or butadiene contents of the feed, but no conclusive data are available. Run-aways are indicated to occur more often with propylene as the olefin being alkylated than with butylenes, but definitely have occurred with butylenes as well.

In general, commercial alkylation plants are usually operated with a used acid discharge acidity of about 90 percent because the operators feel that is about the minimum safe acidity. However, it is known that lower acidities can be used. For example, it is reported by E. C. Hughes, D. G. Stevens, and Franklin Veatch, Industrial and Engineering Chemistry, Vol. 43, No. 6, page l447145l, that butylene alkylation can be operated successfully at 85 weight percent titratable acidity with an acid consumption of around 0.6 pound per gallon of alkylate as compared with 1.2 pounds per gallon at 92 percent acidity.

As pointed out hereinbefore, to quite an extent the run-aways are not understood and are unpredictable. This is especially true the lower the titratable acidity of the system acid. If an abnormally fast drop in acidity is detected before the acidity drops below the safe minimum acidity, the-acidity can usually be brought back to a safe point by increasing the fresh acid feed and/or by decreasing or shutting off the olefin feed. A major difficulty, especially in commercial operation, is that there usually is no continuous monitoring of the acidity, with the result that a matter of hours may elapse between the time a sample is taken and analytical data on acidity are obtained. When the test results are obtained, the acidity already may be so low, for example, about percent, that the acid is no longer an alkylation catalyst. The result is that no matter how much fresh acid is charged, within the capacity of the reactor and settler, and even if the fresh olefin feed is cut out, the acidity cannot be raised to a point at which the acid will again act as an alkylation catalyst. In the past, the remedy has been to dump the acid or remove it from the system. Furthermore, alkylate produced under such conditions is usually high in sulfur, and must be treated for sulfur removal if it is to be used for its intended purpose.

SUMMARY OF THE INVENTION Alkylation unit acidity run-aways are both caused by, and are the result of, the characteristic that the acid remains active for the absorption of olefin after it is no longer active for catalyzing alkylation. There probably is a transition period with a range of acidity, and perhaps with non-uniform acid throughout the reactor and settler, when alkylation and absorption occur simultaneously with a gradual build-up of absorption product till absorption finally becomes the exclusive reaction to the complete exclusion of alkylation. When this point is reached it is futile to try to bring about alkylation by charging fresh acid or by cutting out the olefin feed, since an alkylation catalyst is not present. A further effect of the run-away is that the acid, and also the hydrocarbon to a lesser extent depending on how long the run-away is allowed to run, becomes loaded with alkyl sulfates. Initially the alkyl sulfates will be largely the monoalkyl acid sulfate, but, if allowed to run long enough, the dialkyl sulfates will predominate.

I have discovered that if run-away acid, the overall acid-hydrocarbon emulsion or reaction mixture produced during a run-away, or the alkylate produced and which contains sulfur, is charged to a normally operating alkylation reaction that the acid can be restored to alkylation strength by alkylation of the alkyl sulfates, and the hydrocarbon portion of the reaction mixture can be freed of sulfur by extraction and alkylation of the alkyl sulfates. Alkylate containing sulfur produced during the run-away condition is desulfurized.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a simple two reactor alkylation system, without refrigeration and fractionation facilities, with provision for recycle of run-away reaction mixture from the second reactor to the first reactor.

DESCRIPTION OF PREFERRED EMBODIMENT Although it will become evident from the discussion which follows hereinafter that the present invention can be carried out in numerous ways, it will be helpful to describe in detail the embodiment shown in the drawing.

Referring to the drawing, olefin feed in line 11, isobutane feed in line 12 and fresh sulfuric acid in line 13 are charged to first alkylation reactor 14 which is operated under good alkylation conditions. Reaction mixture from reactor 14 is passed through line 15 to settler 16 in which an acid phase and a hydrocarbon phase form and separate. The hydrocarbon phase containing alkylate product is passed through line 17 to conventional treating and fractionation facilities not shown. A part of the acid phase from settler 16 is returned through line 18 to reactor 14. The rest of the acid phase is passed through lines 18, 19 and 20 to second alkylation reactor 21. Olefin feed through line 22 and isobutane feed through line 23 are also passed to reactor 21 operated under good alkylation conditions. Reaction mixture from reactor 21 is passed through line 24 to settler 25 in which an acid phase and a hydrocarbon phase form and separate. Hydrocarbon phase containing alkylate product is passed through lines 26 and 17 to conventional treating and fractionation facilities not shown. A portion of the acid phase is returned to reactor 21 through line 27. The remainder of the acid from settler 25, which is relatively small in relation to that passed to reactor 11, is discarded as used acid through line 28.

When for some reason a run-away occurs in reactor 21 the system acid drops below alkylation strength and the alkylate product contains dialkyl sulfate which renders it unacceptable as a blending agent of high lead susceptibility. When this happens, fresh olefin feed to reactor 21 through line 22 is stopped. Valve is closed so that reaction mixture does not pass to settler 25. Valve 31 also is closed so that alkylate containing sulfur does not pass to the treating and fractionation facilities. Total reaction mixture from settler 25 via line 27 to reactor 21 and from reactor 21 is passed through line 29 to reactor 14. Olefin feed in line 11 is reduced in an amount equivalent to the olefin in the form of alkyl sulfates passed to reactor 14 through line 29. Used acid is discarded through line 19. Alkyl sulfates in the acid and alkylate are alkylated with the production of sulfur free alkylate and regeneration of acid effective for alkylation.

After all of the contents of reactor 21 and settler 25 are charged to reactor 14, part of the acid from settler 16 through lines 18, 19 and 20 is charged to reactor 21. When sufficient reaction mixture from settler 16 has been charged, isobutane and olefin feed can be resumed to the reactor through lines 22 and 23 as in normal operation before the run-away.

Sulfuric acid alkylation of isoparaffins with olefins is described in many articles and patents, and is widely used commercially. Propylene, butylenes and amylenes are the olefins most commonly used for alkylation of isoparaffins. The reaction is carried out at about 30 to 50 F. with efficient mixing. Strong sulfuric acid of 98.0 to 99.5 percent by weight H 80 olefins and isobutane are continuously charged to a reactor with the isobutane in great excess of the olefin. The reaction mixture is separated into an acid phase and a hydrocarbon phase. Most of the acid phase is recycled to the reactor and a small portion amounting to about 0.3 to 1.0 pound per gallon of alkylate is withdrawn. This small withdrawn portion is referred to as used or spent alkylation acid. It has a titratable acidity in the range of about to 92 percent H SO usually about percent. The composition of the used acid does not vary appreciably with the olefin used, although the amount of the used acid will. In general, the hydrocarbon content of the used acid increases with decreasing titratable acidity. This used acid can be used for fertilizer manufacture, for conventional recovery by burning, or in a recovery process described in US. Pat. Nos. 3,227,774; 3,227,775; 3,234,301; 3,422,164; 3,428,705 and 3,448,168, all issued to Arthur R. Goldsby.

As indicated hereinabove, it is difficult to predict what will cause a run-away or when one will occur. For any given alkylation unit, run-aways do not occur frequently, but when they do, they are serious. When one occurs, steps must be taken to bring conditions back to normal, because the unit becomes inoperable.

The analysis of a run-away acid will vary widely depending on how long the run-away has been allowed to proceed, and how much fresh acid has been pumped at an abnormally high rate into the system in an attempt to reverse the decline in acidity. However, in general run-away acid differs from normal alkylation acid in having a lower titratable acidity, a lower free H SO content and a higher content of alkyl acid sulfate and dialkyl sulfate, and loss of or reduced catalytic activity for alkylation.

Although analyses vary, the following typical analyses will serve to point up the major diflerences between alkylation acid having catalytic activity for alkylation and run-away acid without catalytic activity for alkylation:

Run-away acid, such as the one of the above analysis, can be alkylated with isobutane. In this operation, the alkyl acid sulfate and dialkyl sulfate are alkylated with release of I00 percent H SO The quantity of acid oil complex and water are unchanged and the net effect is to produce acid of about the same composition as the acid before the run-away, or close to the analysis above of the alkylation acid. It actually is a better alkylation catalyst than the system acid before the run-away if a lot of fresh acid has been charged in an effort to reverse the run-away, and excessive side reactions resulting in the production of polymeric oil have not occurred. In addition, if the run-away has been allowed to continue for some time so that the acid phase contains an appreciable concentration of dialkyl sulfate, such as dipropyl and/or dibutyl sulfate, for example 20-80 percent by weight, then the hydrocarbon phase also will contain dialkyl sulfate. The amount of dialkyl sulfate in the hydrocarbon phase will depend on a number of factors, such as temperature and hydrocarbon-to-acid ratio, aside from how long the run-away has continued. At a temperature of about 40-50 F. and a hydrocarbon-to-acid ratio of about 1.0, the dialkyl sulfate content of the hydrocarbon phase can be in the range of about 1 to 20 percent.

In a few unusual cases the run-away acid can be of such poor quality that it cannot be alkylated with a reasonable acid consumption. This can happen if a large amount of isobutylene is reacted with the acid during the run-away or if the temperature is allowed to rise excessively, for example on the order of 75 to 100 F. or higher, so that considerable acid soluble polymeric oil is formed. Poor catalyst acid also can result iffor any reason an abnormal amount of water gets in the acid, either by oxidation reactions or by introduction from outside. However, even under such conditions the hydrocarbon, particularly the alkylate, containing sulfur in the form of dialkyl sulfates can be advantageously alkylated by charging it to an alkylation system.

It may be stated as an approximation or rough rule of thumb, that if the run-away acid contains polymeric oil and water in amounts such that their total quantity after alkylation is greater than the difference between 100 and the desired alkylation system acidity, it is not economically attractive to alkylate it. For example, if after alkylation the acid soluble polymeric oil content is 7 percent and the water content is 4 percent, the sum is 11 percent. The difference between 100 and the desired system acidity of, say 90 percent, is percent. Hence, since the sum of the oil and water of l 1 percent is greater than the 10 percent, this acid should not be alkylated. Similarly, if the acid after alkylation should contain about percent of polymeric oil, it should not be alkylated. Ordinarily, a run-away acid suitable for alkylation will contain after alkylation about 3 to 5 percent of polymeric oil and 2 to 3 percent of water, or a total of oil and water of about 5 to 8 percent by weight. The amountof oil and water can be quite low if an abnormally high amount of fresh acid has been charged during the run-away.

It is desirable and advantageous to alkylate all of the run-away alkylation reaction mixture which contains dissolved dialkyl sulfate. It also is desirable and advantageous to alkylate the hydrocarbon portion of the reaction mixture before it has been through the treating system. In the case of a simple, closed cycle refrigerated unit, as with ammonia or propane, this is the entire reaction mixture, including both the acid and hydrocarbon phases, from the settler. In the case of an effluent refrigerated unit, such as described by A. R.

Goldsby and D. H. Putney, Petroleum Refiner, Process Issue, September, 1955, Vol. 34, No. 9 pages l48-151, this is the reaction mixture going to the settler, the acid or the liquid hydrocarbon from the settler after it has passed through a heat exchanger, and hydrocarbon phase from the settler. In the case of a cascade autorefrigerated unit, such as described by A. R. Goldsby and D. K. Beavon, Petroleum Refiner, June 1959, Vol. 38, No. 6 pages 165-168, this is the entire reaction mixture going to the settler, or the acid and liquid hydrocarbon phases from the settler. In effluent refrigeration a considerable portion of the hydrocarbon reaction mixture is removed as vapor in the refrigeration step, and although this should be sent to alkylation because of its high content of isobutane, it can be charged as a separate stream, if desired, and not as a part of the hydrocarbon phase from the settler.

The liquid hydrocarbon phase from the settler can be subjected to the usual treating step, such as by caustic and water washing, and then alkylated, but is is more advantageous to alkylate it before it has gone through the treating step. The treated hydrocarbon phase also can be subjected to fractionation to remove hydrocarbons other than alkylate, and then only the alkylate bottoms containing dialkyl sulfates are alkylated. However, corrosion and decomposition of dialkyl sulfates can be experienced. In such cases, the alkylate contains so much alkyl sulfates that it is not possible to distill it at atmospheric pressure without excessive decomposition of the dialkyl sulfates.

The run-away acid and/or reaction mixture or just the alkylate can be alkylated in a number of different ways. This will depend to some extent on whether the alkylation unit has two or more reactors with a settler for each reactor, or two or more pairs of reactors with a settler for each pair of reactors, or whether just one reactor and a settler are available.

When a run-away occurs in a unit with a single reactor and settler, it is necessary to cut out the acid and hydrocarbon feeds to the reactor, and pass the reaction mixture from the reactor and the acid and hydrocarbon phases from the settler to tankage. After alkylation conditions have been reestablished in reactor, the reaction mixture or either or both of the acid and hydrocarbon phases from tankage is charged to the alkylation reactor along with fresh acid, olefin feed and isobutane.

The same general procedure is followed with a multiple reactor unit. The fresh feed to that portion of the unit in which the run-away occurred, usually the last reactor in series, is cut out and the reaction mixture pumped to tankage. The material from tankage is then charged to a reactor operating under alkylation conditions, and usually along with at least part of the fresh olefin feed normally charged to the reactor.

In some cases, especially when the run-away has been detected early before too much of the acid has been converted to alkyl sulfates, for example not over about 50 percent, it is possible and advantageous to recycle reaction mixture or acid phase from the runaway reactor or settler to a reactor under alkylation conditions. In this case acid from the stable reactor-settler is discharged into the run-away reactor with no interruption of operation, and the olefin feed to the runaway reactor is cut out until the system acid acidity has been brought back to about percent.

The run-away product representing olefin feed in the form of alkyl sulfates can be alkylated as the entire olefin feed to an alkylation reactor, or it can be alkylated along with all or part of the olefin feed normally charged to the reactor. The same space velocity (defined as gallons per hour of olefin per gallon of acid in the reactor) can be used, or if it is at a reasonable figure and cooling is available, the space velocity can be increased appreciably, especially since it requires only a relatively short time to work off the run-away product. A space velocity of about 0.2 to 0.5 is usually used, although a considerably broader range can be used, for example 0.1 to 1.0.

When a run-away product is alkylated, not as much cooling is required as for molecular olefin, since about 50 percent of the heat has already been evolved in the run-away step. Thus, this enables somewhat higher space velocities to be used without cutting back too much on fresh olefin feed without suffering from too high a temperature with a resulting high acid consumption.

Another feature of my invention is that when runaway alkylation acid is stored without cooling or circulation, it has a tendency to become hot and to decompose with the evolution of sulfur dioxide. Now, with my novel process it is possible to keep the run-away acid within the system under controlled conditions until it is ready for some form of disposition or recovery thereby eliminating the danger of decomposition of the acid with the formation of sulfur dioxide.

Obviously, many modifications and variations of the invention as hereinabove set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

Iclaim:

1. In an alkylation system comprising a plurality of alkylation reaction zones wherein an isoparaflin hydrocarbon normally is reacted with an olefin-based material in the presence of sulfuric acid which flows serially through said reaction zones, said sulfuric acid having catalytic activity for promoting said isoparaffinolefin reaction and having a titratable acidity greater than about 88 percent thereby forming a reaction mixture comprising said reactants, said catalyst and alkylate product in each reaction zone, and wherein the titratable acidity of the sulfuric acid in at least one of said reaction zones uncontrollably falls below catalytic strength accompanied by an abnormal increase in the alkyl sulfate content in the reaction zone and the sum of the polymeric oil and water contents of the acid does not exceed minus the desired system acidity and wherein at least a portion of the alkyl sulfate dissolves in said alkylate product of said reaction mixture, the improvement which comprises stopping the flow of isobutane, olefin and sulfuric acid charge streams to a first reaction zone wherein said sulfuric acid is below catalytic strength, withdrawing the contents of said first reaction zone and passing said contents to a second reaction zone wherein isobutane is being alkylated with olefin-based material, simultaneously reducing the quantity of said olefin-based material charged to said second reactor in an amount equivalent to the quantity of olefin in the form of alkyl sulfates contained in said contents from said first reactor, continuing the alkylation in said second reactor, recharging said first alkylation reaction zone by passing sulfuric acid alkylation catalyst separated from said second zone to said first reaction zone and resuming alkylation in said first zone following the introduction of isobutane and olefin charge streams therein.

2. The process of claim 1 in which said olefin-based material is selected from the group consisting of propylene and butylenes. 

2. The process of claim 1 in which said olefin-based material is selected from the group consisting of propylene and butylenes. 